Nuclear policy in the United States has yet to escape the Cold War’s shadow. In this account of the atomic bomb, a former director of the Defense Threat Reduction Agency makes a case for a reanalysis of the nation’s nuclear needs. “Our nuclear weapons stockpile is optimized for a threat that no longer exists,” Younger writes.

Younger offers a straightforward account of nuclear weapons: how they were developed, how they work and how they forced humankind into constant vulnerability, straddling...